1. Field of the Invention
The present invention relates to a casting method based on a pressure casting process, and a casting apparatus for use in this casting method.
2. Description of the Related Art
As is generally known, a casting process of pouring molten metal or molten alloy (hereinafter referred to occasionally as “molten metal” for brevity) under pressure into a die cavity (hereinafter referred to occasionally as “cavity” for brevity), so-called pressure casting process, e.g. a die casting process, is often used, for example, in casting where light metal or light alloy, such as aluminum or its alloy, or magnesium or its alloy, are used as a material.
However, even the casting using the pressure casting process still involves a problem about defects occurring at a portion of molten metal located in a part of the die cavity relatively far from a pouring gate (for example, typically seen in the most downstream portion of molten metal relative to the pouring gate), such as: so-called “shrinkage cavity” occurring when the portion has a large thickness, so-called “misrun” occurring when the portion has a small thickness, etc. Generally, such defects are caused by the fact that a pressure of molten metal is not effectively transmitted to the portion located a certain distance or more from the pouring gate, or that air in the die cavity is trapped by or entrained into molten metal.
The insufficient transmission of a molten metal pressure in a portion located a certain distance or more from the pouring gate generally arises from deterioration of dies, fluctuation in surface temperature of dies due to changes in working temperature or cooling conditions (amount and/or temperature of cooling water, etc.) of the dies, fluctuation in temperature and/or chemical contents of the molten metal, and/or other factors. The entrainment of air into the molten metal generally arises from fluctuation in molten metal temperature, chemical contents of the molten metal or occluded gases, lowering in degree of evacuation due to deterioration of die-sealing materials, and/or other factors. It is also known that an excessive pouring rate of molten metal into the die cavity is liable to cause the entrainment of air.
While the occurrence of casting defects due to insufficient pressure (or insufficient filling) or air entrainment can be drastically reduced by exactly detecting and controlling the above various factors, the practical use of such measures has not been achieved because of a lot of costs required therefore.
In connection with the above problems about the pressure casting, for example, Japanese Patent Laid-Open Publication No. 2002-103014 proposes a metal injection molding method, which detects a molten metal pressure in a molten metal flow channel on its downstream side relative to a die cavity, and then sets a pouring rate (injection speed) of molten metal into the cavity in accordance with the change of pressure in the molten metal flow channel.
In order to prevent the occurrence of casting defects due to insufficient filling, it is most desirable that the filling of molten metal is completed before the molten metal finishes solidifying, i.e. when the molten metal is still in its fully molten state (i.e. liquid phase) or at least in its semi-molten state (i.e. mixed phase of liquid and solid), to assure the good states of “run” (i.e. to prevent the occurrence of misrun) and to allow the pressure of the molten metal to be transmitted to every distal end of a cast article. Therefore, it is required to fill a die cavity with molten metal at a high pouring rate to a maximum extent. On the other hand, in order to prevent the occurrence of casting defects due to air entrainment, it is also required to keep the pouring rate of the molten metal into the die cavity from excessively high level.
That is, an excellent cast article free from casting defects due to insufficient filling or air entrainment can be obtained, only if a condition for filling a die cavity with molten metal is set in such a manner that the above two conflicting factors are simultaneously satisfied.
However, the aforementioned prior art designed to set the injection speed in accordance with only the detected molten metal pressure can facilitate to suppress the occurrence of defects due to insufficient filling, but has difficulties in stably preventing the occurrence of defects due to air entrainment.
Accordingly, as a solution of the above problems, a casting method and a casting apparatus which simultaneously suppress both the occurrence of defects due to insufficient filling of molten metal and the occurrence of defects due to air entrainment has been desired.